Paradoxically, some microbial infections get a boost from host immune responses. A group of worm parasites called filarial nematodes, which cause river blindness and elephantiasis, adapt their development to maximize reproductive success in response to specialized host immune cells called polynuclear eosinophils. Babayan et al. have found that even the most vigorous eosinophil response is rarely completely protective, and in fact seems to invigorate the worms to reproduce earlier in their life cycle with larger numbers of offspring. The problem lies in current strategies for experimental vaccine development, which rely on the same eosinophil responses that send the reproductive signals to the parasite. Hence, seeking to develop an antifilarial vaccine that does not provide sterilizing immunity may not result in public health benefit.